Thanks for the response Ilian!
Is Mathematica really shipping a libstdc++? I'd be surprised.
No, it is not. This was my misunderstanding.
To match the compiler used for building the kernel, use gcc 4.8.
Earlier today I tried building with gcc 4.8.5 on Ubuntu 15.10, and people tell me that the result does work on Ubuntu 14.04! :-) I am also going to try -D_GLIBCXX_USE_CXX11_ABI=0
